Added option "-s" to str2hex and update hf-referencia.html

$ str2hex -s "/etc/passwd"
0x2f6574632f706173737764

Exemplo:

mysql> select LOAD_FILE(0x2f6574632f706173737764) ;
....
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/bin/false
.....